HYDRAULIC PISTON MACHINE

Abstract: A hydraulic piston machine is disclosed, with a cylinder body (1) and a control counter-plate (4) which engage one another via the intermediary of a control surface, the cylinder body having at least one cylinder which emerges by way of an opening (2) into the control surface, and the control counter-plate (4) has a high-pressure channel (6) and a low-pressure channel (5) which are connected to a high-pressure connection and a low-pressure connection (7) respectively and, upon movement of cylinder body (1) and control counter-plate (4) relative to one another, are passed over by the opening (2), the low-pressure channel (5) having a groove (9) at its end passed over first by the opening (2). It is desirable to be able to operate a machine of that kind also with a hydraulic fluid that has a relatively low viscosity. For that purpose, the groove (9), together with the opening (2), produces a directed jet (12) at least immediately after the start of their overlap.

Description: Hydraulic piston machine The invention relates to a hydraulic piston machine with a cylinder body and a control counter- plate which engage one another via the intermediary of a control surface, the cylinder body having at least one cylinder which emerges by way of an opening into the control surface and the control counter-plate has a high-pressure channel and a low-pressure channel which are connected to a high-pressure connection and a low- pressure connection respectively and, upon movement of cylinder body and control counter-plate relative to one another, are passed over by the opening, the low- pressure channel having a groove at its end passed over first by the opening. In known machines of that kind (US 38 90 883) , such a construction of the low-pressure channel is an attempt to produce a pressure equalisation of sorts in order to avoid problems that can occur during the transition from a relatively high pressure in the cylinder to a relatively low pressure in the low-pressure channel. These problems include in particular the fact that the highly-pressurized hydraulic fluid develops such a great speed immediately after a fluid connection has been established between the opening and the low-pressure channel that it can lead to jetflow erosion in the machine. The grooves are therefore constructed so that they have a throttling effect, so that the speed of the hydraulic fluid can be markedly decreased. For the converse situation, namely, for the high- pressure side, DE 37 25 361 Al describes how such jetflow erosion in the cylinder can be avoided: a channel for a disturbing jet emerges into the groove in such a way that the flow directions of the fluid entering the cylinder from the groove and of the fluid leaving the opening of the channel for the disturbing jet meet. This produces turbulence of the fluid which prevents the fluid striking the cylinder wall at high speed. Both known possibilities work satisfactorily, provided that the hydraulic fluid has a viscosity that corresponds approximately to that of hydraulic oils. If the viscosity of the hydraulic fluid is much lower, however, for example, if the viscosity of the hydraulic fluid corresponds to that of water, the throttling afforded by the groove does not reduce the speed at all or only to an inadequate extent. Increasing the throttling effect is in many cases impossible for reasons to do with construction and manufacturing techniques. The use of a disturbing jet also does not always have the desired success with low-viscosity fluids. This is all the more difficult considering that, as viscosity decreases, in many cases an increase in the mechanical hardness of the fluids can be observed, which in turn increases the risk of jetflow erosion. The invention is therefore based on the problem of guaranteeing as long a service life of the machine as possible even when using low-viscosity hydraulic fluids. In a hydraulic piston machine of the kind mentioned in the introduction, this problem is solved in that the groove, together with the opening, produces a directed jet at least immediately after the start of their overlap. The throttling effect of the groove can therefore here largely be dispensed with. As a consequence, one has to accept a higher speed of the hydraulic fluid emerging from the cylinder into the low-pressure channel. But since the matching of the groove and the opening to one another has ensured that at the instant at which the hydraulic fluid has this high speed a directed jet is produced, this jet can be directed so that it does not cause any significant damage. This can be achieved most easily by making the jet cover as long a path as possible in the low-pressure channel before it meets the wall of the low-pressure channel. The damping, that is to say, the braking of the fluid transported in the jet of fluid, is then effected by the surrounding fluid. As it meets the wall of the low-pressure channel, the fluid of the jet has only relatively little energy left, which is not normally sufficient to cause appreciable damage. The jet also fans out in the surrounding fluid so that it is no longer so fierce. In this connection is it preferable for there to be a path that is larger than 3.5 times the width of the channel between the start of the groove and a projected point of impact of the jet on a wall of the channel. This path is normally sufficient to damp the jet of fluid emerging from the cylinder to a satisfactory extent. When the jet meets the wall, it will then have fanned out and widened to such an extent that it can longer be described as forming a point. The expression "projected point of impact" is intended to mean the point which would exist without damping by the surrounding hydraulic fluid. In an especially preferred construction, several cylinders with corresponding openings are provided in the cylinder body, the path crossing the projection of the opening at least of the next cylinder in the direction of movement. The jet therefore has to pass through the flow of fluid leaving the next cylinder or entering the next cylinder. This effects an additional damping of the jet, that is, the fluid conveyed in the jet, in that the jet is disturbed by the hydraulic fluid emerging from the next cylinder in the direction of movement or the hydraulic fluid entering it. The jet is consequently further deflected, namely, in a direction at right angles to the control surface. As described in DE 37 25 361 Al, the jet is similarly disturbed, or rather the hydraulic fluid undergoes local turbulence, which absorbs more energy. The jet is preferably inclined in relation to the control surface, the groove being correspondingly inclined at its base. The inclination is designed so that the distance between the groove base and the control surface is larger at the start of the groove than at the end of the groove, the end of the groove being located at the transition to the low-pressure channel. In this way, the jet can be prevented from striking the groove base and causing damage there through jet erosion. At the same time, the jet already has the opportunity to fan out in the groove, so that when it enters the low-pressure channel it can immediately be damped by the surrounding fluid and can therefore be braked. A positive side effect is that noise is reduced. The low-pressure connection advantageously has an opening in the direction of movement, at least in the region of the start of the low-pressure channel. This has the advantage that, in motor operation, in the low- pressure channel, at least in the region of its commencement, there exists a flow which is substantially opposite to the jet direction. This leads to further damping or braking of the fluid of the jet, so that further energy can be extracted from the jet as a result. The low-pressure connection can, of course, also be allowed to emerge into other regions of the low-pressure channel as well. The jet is in this connection preferably directed into the low-pressure connection. In motor operation. this does increase the speed of one component of the jet, namely, the component running at right angles to the control surface, because this component acts in the same direction as the corresponding component of the outflowing fluid. But this results in a further deflection of the jet, which more than compensates for that effect. In addition, the path between the start of the groove and the projected point of impact can be further enlarged. The damping and braking effect is consequently further increased. In pump operation, the fluid flow is directed oppositely to this component and thus brakes it. The control counter-plate is preferably completely enclosed by a friction-reducing layer. The friction- reducing layer therefore also lines the walls of the low-pressure channel. A friction-reducing layer of that kind, in particular in conjunction with the low- viscosity hydraulic fluids, has advantages, since these fluids generally have no lubricating properties. A friction-reducing layer of that kind is generally more susceptible to erosion by fierce jets of fluid, but in the present case this is unimportant since provision has been made to ensure that no fierce jets of fluid are able to strike this layer. It is especially preferable for the layer to be formed from plastics material. Examples of plastics materials which may be considered for the layer are, in particular, materials from the group of high-strength thermoplastic plastics materials on the basis of polyaryl ether ketones, in particular polyether ether ketones, polyamides, polyacetals, polyaryl ethers, polyethylene terephthalates, polyphenylene sulphides, polysulphones, polyether sulphones, polyether i ides, polyamide i ide, polyacrylates, phenol resins, such as novolak resins, or similar substances, and as fillers, use can be made of glass, graphite, polytetrafluoro- ethylene or carbon, in particular in fibre form. When using such materials, it is likewise possible to use water as the hydraulic fluid. The invention is described hereinafter with reference to a preferred embodiment in conjunction with the drawing, in which Fig. 1 shows a plan view of a control counter-plate and Fig. 2 shows a diagrammatic cross-section through the corresponding part of a hydraulic machine along the line II-II according to Fig. 1. A hydraulic piston machine, not shown in full detail, which operates according to the axial piston principle, has a cylinder body 1, only the lower end of which is illustrated, in which there are arranged openings 2 by means of which the cylinders arranged in the cylinder body 1 emerge into a control surface 3. The cylinder body 1 lies by way of this control surface 3 in two-dimensional engagement with a control counter-plate 4. The control counter-plate 4 has a low-pressure channel 5 and a high-pressure channel 6 divided into several sections, which channels connect respectively with a low-pressure connection 7 and, not illustrated in detail, a high-pressure connection. The low-pressure channel 5 and the high-pressure channel 6 are of arcuate or kidney-shaped construction and are therefore also sometimes referred to as "control kidneys". On rotation of the cylinder body 1 in the direction of the arrow 8 relative to the control counter-plate 4, the openings 2, and consequently the individual cylinders, come periodically into fluid connection with the high-pressure channel 6 and then with the low-pressure channel 5. The low-pressure channel 5 has for this purpose at its leading end, that is to say, at the end over which an opening 2 passes first, a groove 9, which is shifted radially inwards with respect to a circle 10 defining the radial centre of the low-pressure channel 5 and the high-pressure channel 6. The groove 9 is relatively wide and relatively deep so that it has virtually no, or only a negligible, throttling effect on the hydraulic fluid flowing through. The groove 9 and the section 11 of the opening 2 coming into contact with it first are matched to one another so that immediately after the start of the overlap between the opening 2 and the groove 9 a jet of fluid 12 is produced which is directed so that it has to cover as large a path as possible in the low- pressure channel before it meets a wall 13 of the low- pressure channel 5 and the low-pressure connection 7, which is arranged in a rear flange 14. Over this relatively long path, which is at least 3.5 times the width d of the low-pressure channel 5, the fluid of the jet 12 loses speed considerably. The surrounding hydraulic fluid damps the jet 12 and brakes it. The jet consequently fans out so that it is no longer possible for the wall 13 to be struck by a fierce jet. A projected point of impact 15 at which an unchecked jet 12 would strike the wall 13 is defined merely for the purposes of illustration. The jet 12 passes an opening 2 ' in a leading cylinder in the direction of movement 8. Since this cylinder likewise delivers fluid into the low-pressure channel 5 and receives fluid therefrom, a further damping of the jet 12 is effected here, accompanied by a certain local turbulence of the fluid. The jet 12 is acted on here by a flow component substantially at right angles to its direction. The low-pressure connection 7, which is shown by a dot-dash line in Fig. 1 for the sake of clarity, emerges in the region of the start of the low-pressure channel 5, that is to say, in the region over which the opening 2 passes first. A certain flow component of the fluid flowing out of the remaining openings 2' , which is largely oppositely directed to the jet 12, is therefore produced. This results in a further damping of the fluid, so that the amount of energy required to be absorbed at the wall 13 is no longer significant. The jet 12 is directed into the low-pressure connection 7, that is to say, it is directed (in Fig. 2) downwardly. Generally speaking, the jet 12 has a component that is directed away from the control surface 3. Accordingly, the base 16 of the groove 9 is also correspondingly inclined, to prevent the jet 12 striking the groove base 16. The control counter-plate 4 is completely enclosed by a friction-reducing layer 17 of a plastics material, such as polyamide (nylon) , polytetrafluoroethylene (PTFE) or polyarylether ketone, especially polyether ether ketone (PEEK) . This layer reduces frictional losses between the control counter-plate 4 and the cylinder body 1, even when the hydraulic fluid has no lubricating properties. Although such a layer 17 is relatively susceptible to jet erosion by fierce jets of fluid 12, these are sufficiently damped and braked before they strike the wall 13, so that erosion damage need not in practice be feared.

Claims: Patent Claims 1. A hydraulic piston machine with a cylinder body and a control counter-plate which engage one another via the intermediary of a control surface, the cylinder body having at least one cylinder which emerges by way of an opening into the control surface, and the control counter-plate has a high-pressure channel and a low- pressure channel which are connected to a high-pressure connection and a low-pressure connection respectively and, upon movement of cylinder body and control counter-plate relative to one another, are passed over by the opening, the low-pressure channel having a groove at its end passed over first by the opening, characterized in that the groove (9) , together with the opening (2) , produces a directed jet (12) at least immediately after the start of their overlap. 2. A machine according to claim 1, characterized in that between the start of the groove (9) and a projected point of impact (15) of the jet (12) on a wall (13) of the channel (5) there is a path that is larger than 3.5 times the width (d) of the channel (5). 3. A machine according to claim 1 or 2 , characterized in that several cylinders with corresponding openings (2, 2 ' ) are provided in the cylinder body, the path crossing the projection at least of the opening ( 2 ' ) of the next cylinder in the direction of movement (8) . 4. A machine according to one of claims 1 to 3, characterized in that the jet (12) is inclined in relation to the control surface (3) , the groove (9) being correspondingly inclined at its base (16) . 5. A machine according to one of .claims 1 to 4, characterized in that the low-pressure connection (7) has an opening in the direction of movement, at least in the region of the start of the low-pressure channel (5). 6. A machine according to claim 5, characterized in that the jet (12) is directed into the low-pressure connection (7) . 7. A machine according to one of claims 1 to 6, characterized in that the control counter-plate (4) is completely enclosed by a friction-reducing layer (17) . 8. A machine according to claim 7, characterized in that the layer (17) is formed from a plastics material.

Inventors: JEPSEN HARDY PETER, JEPSEN, HARDY, PETER
Applicants: DANFOSS AS, JEPSEN HARDY PETER, DANFOSS A/S, JEPSEN, HARDY, PETER
Application year: 1994
Main classification: F04B1
IPC codes: F04B1